1. Field of the Invention
The present invention relates to a band recovering device and a telephone set using the same. More particularly, the present invention relates to a band recovering device for recovering the frequency components of, e.g., a voice signal lying in a band lost due to band-limitation during transmission on, e.g., a telephone line, and a telephone set using the same.
2. Description of the Background Art
When a voice signal representative of the voice is transmitted over, e.g., a telephone line, a voice signal lies in a limited frequency band. As a result, frequency components, originally lying in a band excluded by limitation, are suppressed and therefore lost, degrading the quality of the voice signal.
In light of the above, Japanese patent laid-open publication No. 2002-82685, for example, discloses a voice band expanding device for generating virtual frequency components that should lie in the missing band and a device for practicing the same. The voice band expanding device samples an input voice signal with a sampling frequency four times as high as the upper limit frequency component for converting the voice signal to a digital voice signal. The device then uses the folding or shifting of the sampling frequency to fold or shift symmetrically, respectively, voice signal components lying in the low-frequency band of the digital voice signal to the high-frequency side. The device thus expands the frequency band of the input voice signal.
However, as the voice band expanding device described above folds or shifts the voice signal components of the low frequency band to the high-frequency side to expand virtually the frequency band, the device does not recover the frequency band on the basis of the data of the voice signal itself. More specifically, to restore voice signal components lying in the missing frequency band, the device simply makes the input voice signal zero on every other sampling point or inverts the sign of every other sampling data of the voice signal for effecting the folding or the shift to expand virtually the band.